In processing pulverized coal large quantities must be dried to a controlled moisture level. Current drying techniques make use of fluidized beds of coal through which a steady flow of air is passed, which removes moisture. In the presence of high temperatures smaller particles of coal cake and behave as if they were damp, even when dry. Under these conditions the steady state of the fluidized bed is unstable and spouting in the bed takes place. The result is uneven air contact with the particles.
It is known that drying of a solid by an air stream is facilitated if the air stream is in the form of pulses. According to the present invention phase coordinated fluidic oscillators apply pulsed flow of air to a bed of fluidized coal, the flow being spatially oscillatory. Such action provides an agitation of the fluid particles, since at any point of the bed there exists cyclic air velocity rapidly varying in both flow direction and flow amplitude, which significantly reduces drying time, providing in-bed particle circulation to prevent caking and minimizes air flow requirements.
The present system operates by flowing air through phase locked acoustic oscillators, which are so intercoupled that adjacent ones of the oscillators provide jets which move symmetrically about a centerline so that no net transport force is applied to the bed.
If transport of a bed of coal is desired, the pulsed flow deriving from an array of phase locked oscillators is so arranged that all the output jets of the oscillators move assymetrically about a centerline and preferably in phase so that a net transport exists in one direction. The transport rate is then a function of a balance between air momentum, gravitational forces established in terms of a slope or inclination of the bed from the horizontal, and the average size, shape and density of the particles of the bed. Transport can be achieved in beds which are either horizontal or descending in the direction of flow.
The oscillators of the present system are preferrably of the general type disclosed in U.S. Pat. No. 3,432,804 wherein lying between aligned fluid inlet and outlet passages are generally circular chambers serving as resonant cavities. A stream of pressurized fluid issuing from the inlet passage is laterally deflected cyclically so that quantities of fluid are directed to the outlet passage which vary at a cyclic rate equal to twice the frequency of the rate of deflection of the fluid stream. If the outlet passage is flared symmetrically outwardly, the fluid stream sweeps across the downstream end of the passage providing the symmetrical spatially oscillatory flow discussed above. If the outlet passage is flared outwardly in an assymmetrical fashion, the fluid stream sweeps across the downstream end of the passage providing the assymmetrical spatially oscillatory flow discussed above.
In accordance with the present invention, the individual oscillators are phase coordinated by interconnecting the resonant lobes of adjacent oscillators. The oscillators resonate at substantially equal frequencies (or harmonics thereof) so that the interchange of acoustic energy between the resonant lobes or cavities of adjacent oscillators insures phase and frequency coordination. The phases of adjacent oscillators may be the same or out of phase by specified degrees depending upon the length of the passages interconnecting the resonant cavities.
Advantages of the present system are that it operates in extreme environments, reduces transport mechanism complexity, reduces air flow requirement for fluidization, agitates the particles of a bed during fluidication to reduce particle caking and improves heat and mass transfer between the particles and the fluidizing gas.
The oscillators of the present invention may be fabricated from 2 groups of members; a group of upper members and a group of lower members with the members within each group being identical. The members may be extruded, molded or formed by other conventional techniques and are held in place relative to one another by spacers and outer members to form between pairs of said groups of members a power nozzle, an output channel, resonant cavities and resonant cavity interconnections to provide phase coordinated oscillators. The physical structure thus provided is simple and fast and quite economical to construct.